This invention relates to the field of liquid ink printing systems and, more particularly, to methods for improved print quality and color density in color ink jet printing systems.
Ink Jet Printing Systems
A typical ink jet printing system includes a platen for supporting a printing medium such as paper on which an image is to be printed. In some models, a drive means such as a motor is provided for advancing the platen and thereby advancing the paper. In other models, the platen is fixed and drive rollers advance the paper. A print head having one or more nozzles for ejecting drops of ink or a similar liquid printing solution is mounted on a motor-driven carriage. The carriage is moveable along a path transverse to the paper advancement path, and supports the print head with the nozzle(s) facing the platen in an adjacent yet slightly spaced-apart relationship. Platen (paper) position and carriage location combine to position the print head opposite a desired location on the paper.
Ink jet print heads, often called "print cartridges", include at least one and often a plurality of print nozzles. In the latter case, the print nozzles typically form a linear array, arranged vertically with respect to the printing medium (i.e., along a line parallel to the direction of paper advancement). For color ink jet printing, the pen typically includes nozzles for ejecting cyan, magenta and yellow colored inks, called the primary printing colors, or simply "primaries." Four-color systems additionally include nozzles for ejecting black ink. A print head array may also include several nozzles for ejecting each color of ink. In one commercial embodiment, the pen has sixteen nozzles for each of the three (C, M and Y) primaries. A controller, for example, a microprocessor system including associated memory and interfacing electronics, controls paper advancement, carriage motor and print head in a printer.
Printing occurs as the print head traverses across the width of the paper (a "pass"). During each pass all sections of the head are printing, each section printing on a different horizontal band of the paper. Between passes of the print head, the paper is advanced a distance equal to the height of one color section of the head. Paper advances past the printing head from the bottom, passing the cyan primary first. Printing is not completed until all three primaries have passed over the same band on the paper, to allow mixing the primary colors. The drops of ink strike the paper or other medium and then dry to form dots that, when viewed together, Create the permanently printed image. Desired image colors (other than the primaries) are created by combining drops of ink of the primary colors. In other words, drops of two or more colors of ink are deposited on nominally the same location on the paper. The individual dots, typically located on 1/300 inch centers, together form what appear to be reasonably solid fields of a desired color at a comfortable viewing distance.
The fundamental unit of printing area on the paper is commonly referred to as a pixel. The nominal pixel size or spacing is equal to the spacing between nozzles on the print head. The speed of the carriage and the frequency of ejecting drops of ink are controlled to allow depositing successive drops of ink along a horizontal line (or raster) having a spacing similar to the vertical spacing of the nozzles on the print head. If the horizontal print spacing is the same as the vertical spacing between nozzles, each pixel or grid location is nominally square. Other aspect ratios are used as well. The paper may thus be considered as a regular array of pixel areas, for example, consisting of 300 pixel areas per inch in both dimensions.
Alternatively, pixels may be visualized as lying on the nodes of a raster of regularly arranged points in two dimensions. In either case, the pixels are not physically marked on the printing medium other than by dots upon printing. They form a useful convention because they permit an assessment of the dotted image quality actually printed compared to a hypothetical ideal standard pixel array. Since it is the visual appearance of the image that is most important, the use of the pixel location concept also permits comparisons of different methods of forming images using various dot deposition strategies.
Effects of Printing Media and Excessive Amounts of Ink
An important consideration in printing strategies in an ink jet printing system is the intended printing medium. For example, overhead transparencies (OHT) have less affinity for absorbing ink than does a typical paper. As a result, drops of ink deposited on an OHT tend to bead rather than diffuse, as compared to drops deposited on paper. Additionally, the drops of ink deposited on OHT take longer to dry.
U.S. Pat. No. 4,748,453 (Lin et al.) discloses a method of depositing spots of liquid ink upon selected pixel centers on overhead transparencies so as to prevent the flow of liquid ink from one spot to an overlapping adjacent spot. According to that method, a line of information is printed in at least two passes. On a first pass, spots of liquid ink are printed on selected pixel centers in a checkerboard pattern, so that only diagonally adjacent pixel areas are printed. On the second pass, the complementary checkerboard pattern is deposited, thereby completing deposit of ink on all of the pixels in a desired area. That technique allows some drying time between passes.
U.S. Pat. No. 4,617,580 (Miyakawa) is directed to improving color saturation in printed overhead transparencies, by printing multiple drops on each pixel location, each drop being slightly offset horizontally and/or vertically from an adjacent drop.
Printing on paper, however, presents a somewhat different problem. Paper has an affinity for the liquid ink so that substantial absorption and diffusion of each drop of ink generally occurs. On the one hand, diffusion from one drop of ink to a drop that occupies an adjacent pixel area is helpful in achieving color mixing and obtaining a solid appearance. Along a boundary between two adjacent fields of different colors, however, such diffusion results in color bleeding across the boundary, making the boundary appear fuzzy. This is an undesirable result. Other problems associated with printing excessive ink are cockle, curl, wet output, short pen life (for a non-refillable print cartridge), and poor image tone. Excessive ink volumes generally occur as a result of color mixing, described below. However, the present invention, as will be shown, is useful for ink limiting in monochrome applications as well.
Most color ink jet printers form desired image colors by mixing, i.e. by depositing two or more droplets of ink of different primary colors, one over the other, on a selected pixel area of the substrate. The net visual effect is a dot of a secondary color determined by the principles of transmitted or reflected color formation. Black is formed in either of two ways. First, by simply providing black ink, for example in a four-color system. Alternatively, black may be formed by combining all three primaries, C,M and Y. This is called composite black, whereas the first type is true black. True black is indicated by the letter K (to avoid confusion with the color blue). Overprinting three drops of ink on each pixel area results in high ink coverage for a solid appearance. However, the high ink volume (300%, where one drop per pixel defines 100%) also leads to substantial bleeding across color field boundaries, resulting in poor image quality.
Additionally, certain medium types and environmental conditions have undesirable effects on print quality. Specifically, these adverse effects are bleeds from darker colors into lighter ones and bronzing (a usually bronze colored shiny surface) of composite black. High humidity environments increase bleed and bronzing. Some types of plain paper are prone to bleed and bronzing even in normal (air conditioned office) environmental conditions.
Gamma correction is not sufficient to control most of the aforementioned problems. Although gamma correction does affect the amount of ink deposited in intermediate tones, the darkest portion of the image are unaltered by gamma correction. It is in the darkest portion of an image, however, where the excessive ink problem is most acute.
U.S. Pat. No. 5,012,257 to Lowe et al. describes a "superpixel" printing strategy to reduce bleed across color field boundaries. That strategy limits printing to no more than two drops of ink per cell or pixel, and no more than a total of three drops per superpixel, a super pixel consisting of a 2'2 array of pixel cells. This strategy controls bleed, but at a penalty in terms of color and spatial resolution.
The need remains, therefore, to improve print quality in liquid ink printing systems. A need further remains to provide good printed image quality across a variety of environmental conditions and print media. Improved print quality requires good color saturation, while avoiding excessive amounts of ink.